Hi,
On Sun, Jun 11, 2000 at 11:50:44PM +0200, Xuan Baldauf wrote:
>
> Was it ever considered to log diffs rather than full blocks? Most block operations
> need to know the previous data and therefore are loaded. If you just change one bit or
> two in a bitmap, the full block does not need to be changed. I think this would
> dramatically reduce the size of the journal traffic, and it may significantly increase
> the log performance.
It's very much an early design decision in the development of any
journaling mechanism. The logical journaling you describe can be
more compact but requires more CPU time to maintain. The savings
are not as great as you'd expect, though --- if you are allocating
a lot of data, then physical logging doesn't bloat the log, because
you tend to be allocating many blocks from the same bitmap and so
you only need to log that bitmap block once for the whole
transaction. In general it is easier to merge changes in this way
for physical logging than for logical logging (which isn't to say
you can't do it for logical logging, just that physical logging
gives you that benefit for free).
Cheers,
Stephen
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